This invention is directed to an avalanche photodiode useful in high-speed optical fiber applications. More particularly, the invention is related to a planar hetero-interface avalanche photodiode fabricated by wafer bonding and having a high and controllable electric field in both the absorption and multiplication regions.
Ever increasing demand for higher-throughput telecommunication and data-transmission are more than met by the immense information carrying capacities of optical fibers. The deployment of optical fiber transmission systems and their emerging penetration into local area networks has increased the need for high performance (low noise, high speed) and cost effective interfaces between the optical fiber and electronics. A data rate in excess of 40 Gb/s per channel is presently an important goal for the component industry.
Due to their optical absorption characteristics, InGaAs/InP p-i-n and avalanche photodiodes have formed the basis for photodetectors in optical communication systems operating in both the 1.3 xcexcm and 1.55 xcexcm transmission windows. Avalanche photodiodes (APD) having separate absorption and multiplication regions offer higher sensitivity detection in long wavelength optical communication receivers as compared to conventional p-i-n diodes because of the internal gain provided by impact ionization and carrier multiplication. Fiber optic receivers exploit the internal multiplication gain of avalanche photodiodes to achieve substantial improvement in sensitivity as compared to p-i-n photodiode-based receivers. However, the gain-bandwidth product of InP-based avalanche photodiodes is limited because the electron ionization coefficient is nearly identical to the hole ionization coefficient.
The ternary semiconductor In0.53Ga0.47As, lattice matched to InP, is the preferred material for integrated fiber-optic telecommunications receivers. The high absorption of light in the wavelength range between 1.3 xcexcm and 1.5 xcexcm, the high carrier mobility and saturation velocities make InGaAs the material of choice for efficient, high-speed photodiodes. Most of today""s APDs use InGaAs as material for the absorption region and InP as material for the multiplication region. However, since the electron and hole ionization rates in InP are substantially identical, low-noise and high-speed operation is difficult to achieve. APDs made of InGaAs/InP of many different designs have been reported, including monolithically integrated optoelectronic circuits.
Silicon, on the other hand, has dissimilar electron and hole ionization rates and hence is the material of choice for low-noise and high-speed operation, for example, for high-performance reach-through APDs. Unfortunately, silicon does not absorb light in the optical regions used for optical telecommunications. High-performance InGaAs/Si composite APDs are difficult to fabricate since high-quality InGaAs cannot be grown directly on silicon due to a large mismatch of about 8% in the respective lattice constants.
Wafer fusion has recently been shown to overcome some of the intrinsic limitations of APDs based on InGaAs/InP and directly grown InGaAs/Si heterostructures. The use of silicon as a multiplication region and III-V compounds as an absorption region creates highly-efficient photodetectors with potentially high gain-bandwidth products and low-noise.
Prior art hetero-interface APDs were built on Si substrates and have a mesa-type structure to limit the active area and avoid a premature avalanche breakdown in the edge regions of the photodiode. Mesa-type structures produced on Si substrates, however, have several drawbacks: they require a more complex fabrication process and have a higher dark current due leakage along the sidewalls; and stress within the relatively thin III-V layers fused to silicon substrates tends to induce defects in the III-V layers. Moreover, since the substrate of conventional APDs tends to be doped lower, than the multiplication region, the electric field extends in a direction opposite to the absorption layer. The geometry of conventional APDs is therefore not suitable for controllably producing a high electric field within the absorption region.
A semiconductor hetero-interface APD using wafer fusion is disclosed, for example, in U.S. Pat. No. 6,074,892, which describes avalanche photodetectors constructed by two different methods, one using a single fusion step and another using two separate fusion steps. In the single fusion method, an In0.53Ga0.47As/InP wafer was fused to an epitaxial silicon layer grown on a silicon substrate having a shallow p-type ion implant at its surface. After the fusion step the InP substrate was removed.
In the two step fusion method, first an intrinsic In0.53Ga0.47As layer on an InP substrate was fused to an intrinsic epitaxial Si layer grown on an n+substrate with a shallow p-type ion implant at its surface. After the first fusion step, the InP substrate was selectively removed leaving only the InGaAs epitaxial layer. A second p+doped In0.53Ga0.47As layer on an InP substrate was then fused to the first InGaAs layer and the InP substrate subsequently removed. In both fusion methods, the Si surface was implanted with a 10 keV, 1.3xc3x971012 cmxe2x88x922 dose of boron atoms, resulting in a shallow p-type layer with a thickness of a few tens of nanometers.
For an avalanche photodiode to achieve high frequencies, the electric field in the absorption layer must be carefully controlled: it should be high enough to achieve a high carrier velocity, but smaller than about 100 kV/cm to avoid tunneling in the InGaAs region.
To fulfill this condition, four parameters of the prior art APD had to be optimized: the thickness and doping concentration of the epitaxial i-InGaAs layer, and thickness and doping concentration of the implanted p+Si layer. The thickness and doping concentration of the epitaxial i-InGaAs layer can be suitable controlled by selecting the epitaxial growth parameters. On the other hand, the thickness and doping concentration of the implanted p+Si layer, which are more critical, are also more difficult to control: if the doping concentration of the p+layer is too high, then the electric field will not be able to penetrate in the absorption region, thus reducing the photodiode bandwidth. If the p+layer is too thick, then a high bias voltage is needed to entirely deplete the p+layer. The required high bias voltage can be significantly higher than the voltage at which breakdown occurs. This structure is therefore very sensitive to process variations because of the stringent requirements on the shallow implanted p+silicon layer.
It would therefore be desirable to produce an APD that has a wider processing latitude, is easier to manufacture, while still providing a high electric field in the multiplication and absorption regions that is less than the breakdown field.
Indium phosphide (InP) transistors are superior in terms of current density, breakdown voltage, and electron velocity to silicon transistors. It would therefore also be desirable to monolithically integrate high speed low-noise transistors with optoelectronic circuits to achieve high data rates.
The invention is directed to a planar avalanche photodetector (APD) fabricated by wafer-bonding. According to one aspect of the invention, an absorption layer of a second conductivity type with a doping concentration substantially less than a doping concentration of the first semiconductor is applied on a first semiconductor. A second semiconductor of the first conductivity type is wafer-bonded to a surface of the absorption layer opposite to the first semiconductor to form a multiplication layer. A region of the second conductivity type is then formed on a surface of the second semiconductor opposite to the wafer-bonded absorption layer to provide an electrical contact. According to another aspect of the invention, a semiconductor device includes
a first semiconductor of a first conductivity type, an absorption layer of a second conductivity type which is disposed on the first semiconductor and has a doping concentration substantially less than a doping concentration of the first semiconductor, and a second semiconductor of the first conductivity type wafer-bonded to a surface of the absorption layer opposite to the first semiconductor and forming a multiplication layer. The multiplication layer has a layer thickness that is significantly less than a thickness of the first semiconductor and a doping concentration that is greater than the doping concentration of the absorption layer. A region of the second conductivity type is formed on the second semiconductor, wherein the region has a doping concentration that is substantially greater than a doping concentration of the second semiconductor.
The above arrangement of the layers constituting the APD achieves a controllable high electric field in both the absorption and multiplication regions. To this aim, the doping concentration of the multiplication layer adjacent to the absorption layer is set at a lower level than that of the first semiconductor, while having the same conductivity type.
Embodiments of the invention may have one or several of the following features. The first semiconductor can be InP or InGaAs, with the absorption layer being made of InGaAs. However, other III-V materials, such as (AlGaIn)N, (AlGa)As, etc., may also be used. The second semiconductor can be silicon, germanium or another material suitable for providing avalanche gain. The first semiconductor can have a doping level of between 1018 cmxe2x88x923 and 1021 cmxe2x88x923. The first conductivity type can be p-type (n-type) and the second conductivity type can be one of intrinsic or n-type (p-type). The thickness of the multiplication region can substantially identical to the thickness of the absorption region to provide an optimum performance. The respective thickness of the absorption and multiplication region is selected to be between 0.1 xcexcm and 10 xcexcm, preferably between 0.5 xcexcm and 2 xcexcm. The second semiconductor that is wafer-bonded can either have a reduced thickness or can be thinned down to the desired thickness after wafer-bonding. The region of the second conductivity type formed in the second semiconductor can at least partially laterally surrounded by a guard ring. A second semiconductor device can be integrated with the APD on the first semiconductor to produce an integrated optical transceiver.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments and from the claims.